Multiphase heat transfer analysis of air expansion process in an integrated porous media-liquid piston system

Liquid piston gas compressors/expanders (LPGC/E) have been proposed as a key element within isothermal compressed air energy storage (CAES) systems, due to their superior heat transfer between the working gas and the liquid piston. With improved thermal management, the LPGC/E significantly lessen temperature rise during compression and delivers the required heat during expansion, thereby enhancing round-trip efficiency compared with conventional CAES, which often relies on natural gas combustion. This paper presents a computational fluid dynamics (CFD) analysis examining the highly coupled flow and thermal fields during a high-temperature expansion stroke within an LPGC/E. Further analysis of the LPGC/E integrated with porous media inserts demonstrates the impact of inserts on promoting a near-isothermal process and improving efficiency. In the baseline cases without porous inserts, simulations were conducted under two different thermal boundary conditions at the cylinder wall: Case (i), with the wall maintained at a constant ambient temperature, and Case (ii), with the wall assumed adiabatic. Results indicate that Case (ii) achieves the highest expansion efficiency of 82.4%, compared to 79.7% in Case (i), where wall cooling occurs. For the adiabatic cylinder wall, further simulations were performed with porous media inserts under two initial thermal conditions: Case (iii), where the porous inserts and air are in thermal non-equilibrium, and Case (iv), where the two regions are in thermal equilibrium. The findings show that Case (iii) yields an expansion efficiency of 76.1%, whereas Case (iv) significantly enhances efficiency to 88.1%, outperforming the corresponding baseline (no-porous) case. Expansion performance is improved in Cases (ii) and (iv) by reducing heat transfer within the system, underpinning the importance of mitigating the temperature reduction and striving to achieve a near-isothermal process. These findings provide a foundation for developing more efficient LPGC/E-based isothermal CAES systems, offering guidance on optimising thermal management techniques along with porous media integration for practical energy storage applications. • Numerical analysis of air expansion in a liquid piston-driven device is performed. • Coupled flow and thermal fields are examined within a porous media integrated unit. • Baseline case with adiabatic cylinder wall yields expansion efficiency of 82.4%. • Introducing porous inserts under thermal equilibrium raises efficiency to 88.1%. • Thermal non-equilibrium within porous region reduces expansion efficiency to 76.1%.